Engine pumping work may be reduced to increase engine efficiency by operating an engine at higher intake manifold pressures. However, at least for spark ignited engines, it is desirable to regulate the amount of air entering the engine so that the engine air-fuel ratio will not be leaner than is desired, or so that the engine may not produce more than a desired amount of torque. Higher intake manifold pressures can be achieved while regulating the amount of air entering the engine by closing intake valves late. Closing the intake valves late allows air that enters cylinders to be pushed back into the intake manifold during the compression stroke. In this way, intake manifold pressure is increased while cylinder air charge is regulated to less than full load cylinder air charge.
Operating the engine at higher intake manifold pressures provides challenges that were not foreseen when engines where operated with high levels of vacuum in the engine intake manifold. One challenge is to provide sufficient flow from a canister storing fuel vapors to the engine when the engine intake manifold is at a relatively high pressure. If flow from the fuel vapor storage canister to intake manifold is too low, fuel vapors may spill from the canister to ambient air.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for purging fuel vapors, comprising: supplying fuel vapors to an engine via a storage canister and a purge valve; and adjusting an engine valve timing up to and not exceeding a timing where a sonic flow occurs between the storage canister and the engine in response to a concentration of hydrocarbons flowing from the storage canister to the engine.
By adjusting engine operation to provide sonic flow between the canister and the engine while at the same time limiting valve timing to not exceed a timing that provides sonic flow between the canister and the engine, it may be possible to operate the engine efficiently even when purging fuel vapors from the canister to the engine. For example, intake valve timing of an engine operating with late intake valve closing may be retarded to an extent where intake pressure is low enough to provide sonic flow between the canister and the engine, but not where intake manifold pressure is substantially lower than an intake manifold pressure that provides sonic flow between the canister and the intake manifold. In this way, the engine may be operated at a higher engine intake manifold pressure that provides sonic flow between the canister and the engine. Further, in one example, the engine intake manifold pressure that provides sonic flow between the canister and the engine is adjustable to account for changes in barometric pressure. Thus, valve timing can be advanced or retarded as the altitude at which the engine operates changes so that sonic flow between the canister and the engine intake manifold may be provided.
The present description may provide several advantages. In particular, the approach may allow the engine to operate efficiently while providing a high flow rate between a fuel vapor storage canister and the engine. Further, the approach can increase flow of fuel vapors from the canister to the engine when a concentration of fuel vapors stored in the canister is determined to be increasing. Further still, the approach may reduce the possibility of fuel vapors escaping from the canister to atmosphere.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.